1. Field of the Invention
The present invention relates generally to web printing presses and more particularly to a method and device for minimizing the effects of web fanout.
2. Background Information
A lithographic web printing press prints typically prints four colors on a web of material such as paper. As the web passes through blanket cylinder of the printing units, it becomes moist, which can lead to an expansion of the web. The expansion of the web in the axial direction of the printing cylinders is known as web fanout. If fanout occurs, the print or images on the web thus expand slightly as the web passes the first printing unit. The second printing unit needs to be properly registered with respect to the web images or print, so that, for example, a second color is applied as desired over an image already printed by the first printing unit.
Because the image printed by the first printing unit is subjected to fanout, the second or subsequent printing units may not be properly registered, which can lead to misregister of the image.
For some printing applications, multi-plate plate cylinders are used. These plate cylinders have a plurality of axially-spaced printing plates. For proper register of each printing plate, the plates may be movable independently of one another.
U.S. Pat. No. 4,207,815 purports to disclose a two-plate plate cylinder. One plate fits on a large diameter portion and another plate fits on a tube rotatable and axially movable with respect to the large diameter portion. The tube is fastened to a stepped shaft. Helical gearing is provided to set the axial and circumferential register of the plates. This arrangement requires a complicated, double-diameter plate cylinder and complicated gearing, as well as only being applicable to a two-plate configuration.
The device of the ""815 patent also has the disadvantage that no sensors appear to be present to read the different sections of the web, e.g., the front and back of the web. The device also has the disadvantage that a complex helical gear mechanism controls the circumferential and lateral displacement of the shell, thus, the device is more difficult and time consuming to repair.
U.S. Pat. No. 5,383,393 purports to disclose a multicolor lithographic rotary press comprising a plurality of printing sections arranged along a traveling line of a paper web, a plurality of register adjusting means, a paper stretching means, and a plurality of width adjusting means. Each of the printing sections further includes at least one divided plate cylinder, each divided section of which is independently moved in the axial direction and/or circumferential direction. The device of the ""393 patent further discloses a register adjusting means mechanically connected to each of the divided plate cylinders in the printing sections, and includes an adjusting mechanism for actuating the divided sections in response to the control unit connected to a sensor for detecting the lines and images printed on the paper web by each of the printing sections.
The device of the ""393 patent has the disadvantage that multiple sensors are not present to read the web at differing places, thus, determination of fanout is hampered. Furthermore, the device does not have a third shell and thus, the ability to adjust to differing fanout widths of the web is limited. The device also possesses the disadvantage that in order to compensate for fanout, the device changes the width of the web rather than the displacement of the printing plates. The change in the width of the web may result in the process not working well for all paper types. Moreover, the web width adjusting device appears complicated.
An object of the present invention is to provide for a device and method for automatic adjustment of a web printing press by use of sensors to minimize the effects of fanout. An additional or alternative object is to provide a device and method for adjustment of a web printing press by use of a cylinder having one central portion and two axially movable shells on either side of the center portion.
The present invention provides a printing cylinder comprising:
at least one web fanout sensor for reading a web, the fanout sensor having an output;
a first rotatable section having a central image area on the surface, and a first and second end;
a first shell disposed circumferentially about the first end; and
a second shell disposed circumferentially about the second end, the first and second shells having image areas on the surface and being movable axially with respect to the central image area as a function of the web fanout sensor output.
With the first and second shells movable axially with respect to the central image area of the present invention, more precise adjustments can be made to the printing surface to minimize print defects caused by fanout of the web.
The central image area may carry one or two plates, and the shells one or two plates each.
Specifically, if web fanout is occurring, the first and second shells can be moved axially away from the central image area. While this movement will not perfectly correct the problems resulting from fanout, it can reduce the effects of fanout, especially for example when used with newspapers where a plurality of pages are positioned across the web.
Preferably, the at least one web fanout sensor includes two web fanout sensors on one surface of the web. The sensors preferably are located near edges of the web, but may be located in an unprinted area near the center of the shell.
At least one register sensor for proper overall registration of the printing cylinder preferably is also included. The at least one register sensor preferably includes two register sensors located in a center of the web, with a first register sensor on a first surface of the web and a second register sensor on a second surface of the web.
The central image area and the first and second shells may be laser-writeable. Laser writeable shells provides the advantage that a desired image can be changed without having to physically remove a portion of the shell, nor waiting for erasing and rewriting the image; thus, the press downtime can be reduced.
The central image area and the first and second shells each also may further comprise a gap to insert a printing plate. The gap has the advantage that a change to the desired image can be made by first removing the existing printing plate and then modifying and replacing the existing plate or adding a plate with a different image.
The present invention also provides a printing press comprising:
at least one first sensor for detecting register marks;
at least one second sensor for detecting fanout; and
a rotatable cylinder with a first section axially adjustable with respect to a second section.
With the first and second sensors, the amount of fanout can be automatically detected and the effects compensated for by axially adjusting the first and second sections. The adjustment provides the advantage of more efficient printing.
The first sensors preferably detect register marks printed by the central image area, and the sensor fanout marks printed by the shells.
The second sensors may comprise photocell detectors or video detectors. The photocell or video detectors may allow comparison of the location of one printed image to one that had been previously printed; thus, the amount of fanout can be detected without using marks on the web. However, the use of fanout marks on the web such as cross-hairs, is preferably.
The present invention further provides a method for correcting fanout in a web printing press comprising:
sensing register marks to provide overall circumferential and/or lateral register adjustments of a printing cylinder;
sensing a section of the web other than the register marks to provide fanout information; and
changing a lateral position of a first part of the printing cylinder with respect to a second part of the printing cylinder as function of the fanout information.
Preferably the first part is a shell and the second part an image area of a cylinder, the shells being moved axially.
The shells may be moved in a manner similar to that disclosed in co-owned U.S. patent application Ser. No. 09/627,639, which is hereby incorporated by reference herein.